This invention relates to the topic of geophysical logging, particularly logging of a petroleum well by means of guided electromagnetic waves. What is being logged is electrical properties and changes in these, preferably in rocks situated along a metallic string in a drilled well.
The metallic string may be a drillstring or a production tubing. The electromagnetic parameter is the wave impedance between the electrically conductive string and the surroundings. The string""s surroundings comprise a cylindrical cavity generally axially parallel with the string, and more or less porous and permeable geological (mainly sedimentary) layers of varying dielectrical permittivity combined with varying resistivity (conductivity). Due to their porosity the geological strata always contain more or less water. Sediments consisting of sandstones or carbonates have low electrical conductivity in their own, depending on the mineral composition, and schists have a somewhat higher electrical conductivity due to chemical composition and the ionic structure. Water is a polar liquid and may dissolve salts, e.g. NaCl, CaCl2, NaFl which easily form ions Na+, Ca2+, Clxe2x88x92 etc. This water is electrically conductive with resistivity roughly around 0.01 xcexa9m-1 xcexa9m, depending on the amount of dissolved salt and the ionic valence. Oil and gas does not dissolve corresponding amounts of salt. Rocks containing oil or gas have much less electrical conductivity, thus larger resistivity, crudely estimated to be in the range of 101 xcexa9m-103 xcexa9m.
By production from a well is meant to take out liquids and gases. During production of an oil-bearing well one normally does not want to produce water. Due to the liquid density one will, in a liquid trap, e.g. a synform or a fault, encounter gas, oil and water as counted from above in a petroleum-bearing zone in a well. The boundary surfaces in an undisturbed oil/gas/water zone are usually horizontal. The well may contain several petroleum-bearing zones. When taking out oil and gas the boundary surface between oil and water will rise. This boundary surface is called the oil/water contact, hereafter called the OWC: Oil/Water Contact. Due to the liquid not being entirely ideally fluid, but has a limited permeability in the porous rock, the OWC will not be horizontal, but more or less shaped as a curved surface. The shape of the liquid surfaces is determined by the relative viscosities for gas, oil and water and the local permeabilities in the rocks. It is desirable to monitor such liquid surfaces during production of oil or injection of water, natural gas, CO2 or other fluids in the reservoir.
Radar-similar detection in boreholes is known, both by means of pulsed electromagnetic waves and continuous coherent electromagnetic waves. The electromagnetic waves are generated by a signal generator and emitted by means of a rod-shaped or ring-shaped dipole antenna, e.g. fixed to a logging sonde lowered in the borehole. Such borehole radars are described in U.S. Pat. No. 5,530,359: xe2x80x9cBorehole logging tools and methods using reflected electromagnetic signalsxe2x80x9d, describing a directionally sensitive logging sonde having a transmitter antenna and several rod antennas arranged azimuthally about the sonde""s vertical axis.
U.S. Pat. No. 4,504,833: xe2x80x9cSynthetic pulse radar system and methodxe2x80x9d describes a xe2x80x9cgeoradarxe2x80x9d emitting pulsed signals consisting of a series of selected frequencies emitted simultaneously. The system is arranged for a mobile platform and is arranged for sending electromagnetic waves down into geological formations and for detecting electromagnetic waves being reflected from impedance contrasts inside the geological formations.
The patent publication expected to be closest to this invention is U.S. Pat. No. 5,552,786: xe2x80x9cMethod and apparatus for logging underground formations using radarxe2x80x9d concerning a method to determine wave velocities for the radar waves propagating from the transmitter antenna to the receiver antenna xe2x80x9cdirectlyxe2x80x9d via the rocks being closest to the borehole. The wave velocities derived from the measurements between transmitter and receiver antenna (a few feet) is used for the geophysical interpretation of the geological structures around the borehole.
The above mentioned problems are solved by means of a device and a method according to this invention which in a preferred embodiment is a device for detection of changes in resistivity or dielectrical properties due to changes of fluid composition in the near-well area 0-500 m about a well 1 in a geological formation 9, comprising an electrically conductive tubing string 4, e.g. a liner pipe or other fixedly mounted tube or openhole completion in the well 1.
A preferred embodiment of the present invention comprises a device for detecting changes in resistivity or dielectrical properties in the near-well area around a well in a geological formation, comprising an electrically conductive tubing string having a linear dimension, an electrical energy source, a signal generator electrically coupled to the electrical energy source, the signal generator being capable of generating electrical signals, a transmitter antenna coupled to the tubing string in a fixed first position and electronically coupled to the signal generator, the transmitter antenna configured to emit electromagnetic waves guided along the tubing string, a receiver antenna for receiving electromagnetic waves along the tubing string coupled to the tubing string in a fixed second position a distance along the linear dimension apart from the fixed first position of the transmitter antenna, a device for receiving signals induced in the receiver antenna by received electromagnetic waves, a signal processing device capable of processing the received signal and providing a processed signal output, and a communication device electronically coupled to the signal processing device, the communication device being capable of transmitting signals representing the processed signal output and receiving a control signal.
Another preferred embodiment of the present invention comprises:
a) an electrical energy source 24,
b) a signal generator 22 for generation of electromagnetic signals 25 to
c) at least one transmitter antenna 2 for electromagnetic waves 26, arranged preferably above an oil/water contact, in a fixed first position on the tubing string 4, and arranged so as to guide the electrical waves along the tubing string 4,
d) one or more receiver antennas (81,82, . . . , 8n) for guided electromagnetic waves along the tubing string 4, arranged in other fixed positions along the tubing string 4,
e) devices 80 for receiving signals (851,852, . . . , 85n) induced in the receiver antennas (81,82, . . . , 8n) due to wave impedance gradients along the tubing string 4 in rocks or fluids in the geological formation 9 and in the electrically conductive tubing string 4,
f) signal processing devices 82 for processing of the received signals (851,852, . . . , 85n),
g) communication devices 100, 200 for transmission of signals 105 representing the electrical signals (851,852, . . . , 85n), and for receiving control signals 205.
Additional features by the invention are defined in the enclosed patent claims and described in the detailed description below.